Hitherto, it has been customary to calibrate a water meter in a test rig before the meter is installed in a pipe. However, regulations issued by the European Commission now require that a water meter be periodically calibrated in situ, that is, after the meter has been installed in a pipe.
In principle it would be possible to effect in situ calibration using a so-called meter prover, an apparatus conventionally used to calibrate flow meters in oil pipe lines.
Meter provers marketed by numerous companies comprise a by-pass loop which is coupled to the pipe-line downstream of the meter under test. Flow passes through the flow meter and is then diverted through the by-pass loop causing a piston or sphere to be displaced within a calibrated chamber. Flow of a known volume of fluid through the by-pass loop causes the piston or sphere to be displaced between a pair of detectors which are spaced apart from each other by a predetermined distance. By gating the outputs of the detectors, and measuring the time taken for the piston to be displaced therebetween, the flow rate through the meter prover, and so through the flow meter under test, can be accurately determined.
The meter prover gives an absolute measure of flow rate which is independent of parameters such as viscosity, density, temperature and pressure, and provides a very accurate calibration to within 0.02%.
Although meter provers are well-suited to calibrate flow meters in oil pipelines they do not offer a viable solution to the problems presented by in situ calibration of water meters.
A meter prover is a bulky apparatus; indeed, a meter prover suitable for calibrating a water meter installed in a 4" (10.24 cm) pipe would need to be transported on a heavy duty truck and is prohibitively expensive. Furthermore, in many situations it would be difficult, if not impossible, to manoeuvre the meter prover into close proximity with the water pipe.